Polypropylene is a well-known article of commerce, and is utilized in a wide variety of applications which are well known to those of ordinary skill in the art. Polypropylene is utilized widely in many fiber, fabric, or similar product applications. However, it is generally known to be deficient in applications that require high softness such as nonwoven fabrics for disposable garments and diapers. For such soft end-use fiber and fabric applications, macromolecules with a statistical placement of propylene and ethylene monomer units (hereinafter random copolymers) have come into use since they can be processed into fibers and fabrics that exhibit improved softness and drape characteristics in comparison to fibers and fabrics made from homopolymer polypropylene.
Random propylene-based copolymers have long been used in the making of nonwoven spunbond fabrics. In a typical spunbond process a random propylene copolymer resin in granular or pellet form is first fed into an extruder, wherein the resin simultaneously is melted and forced through the system by a heating melting screw. At the end of the screw, a spinning pump meters the melted polymer through a filter to a die (hereinafter the spinneret) having a multitude of holes (hereinafter capillaries) where the melted polymer is extruded under pressure through the capillaries into fibers. The fibers exiting the spinneret are solidified and drawn into finer diameter fibers by high-speed air jets. The solidified fibers are laid randomly on a moving belt to form a random fibrous, mesh-like structure known in the art as a fiber web. These random copolymers have been traditionally made by adding small amounts of ethylene, typically 0.5 to about 6.0 weight percent based upon total polymer weight, in the reacting medium comprising propylene and a catalyst that is capable of incorporating the ethylene monomer into the macromolecule chain, to thereby reduce the overall crystallinity and rigidity of the macromolecule. Random copolymers, because of their lower crystallinity and rigidity, are preferred over homopolymer polypropylene in fiber and fabric applications that require enhanced softness. However, a number of practical limitations have restricted the application of random copolymers in soft fiber and fabric uses. Accordingly, patent literature has discussed means of addressing these problems.
U.S. Pat. No. 6,218,010 relates to an ethylene-propylene copolymer alloy which is suited for making fibers and nonwoven spunbond fabrics having softness at economically acceptable processing conditions. The alloy comprises a random copolymer having an ethylene content of from about 1 to about 5% by weight in an amount of from about 40 to about 90% by weight of the alloy; and a second ethylene-propylene copolymer having an ethylene content of from about 5 to about 40% by weight, in an amount of from about 10 to about 60% by weight of the alloy. The copolymer alloys are described to be prepared by a multi-reactor process comprising a first stage of polymerizing a mixture of ethylene and propylene in single or plural reactors, in the presence of a catalyst system capable of randomly incorporating the ethylene monomers and/or alpha-olefin into the macromolecules to form the random copolymer, and a second stage of, in the further presence of the random copolymer containing active catalyst, polymerizing a mixture of ethylene and propylene in single stage or in plural stages to form the second ethylene-propylene copolymer.
Fibers comprised of ethylene-propylene polyolefin alloys that are capable of preparation by both melt blown and spunbond processes are proposed in U.S. Pat. No. 6,342,565. These fibers are comprised of a soft, set-resistant, annealed fiber comprising a blend of polyolefins, said blend including: a) a first polymer component that is a propylene-ethylene copolymer having 80 weight percent or greater propylene, and having 20 weight percent or less ethylene, said first polymer component present in said fiber in the range of from 75-98 weight percent, based on the total weight of said polyolefins, and said first polymer component having a melting point as determined by differential scanning calorimetry (DSC) in the range of from 25-70° C. and a heat of fusion less than 25 J/g; and b) a second polymer component present in said fiber in the range of from 2-25 weight percent based on the total polymer in said fiber, wherein said second polymer component is a stereoregular isotactic polypropylene (iPP), having a melting point as determined by DSC greater than 130° C. and a heat of fusion greater than 120 J/g.
U.S. Pat. No. 6,635,715 describes blends of a first isotactic polypropylene homopolymer or copolymer component with a second alpha-olefin and propylene copolymer component, wherein the first isotactic polypropylene component has a melting point above about 110° C., preferably above about 115° C., and the second copolymer has a melting point between about 25° C. and 105° C. The blends are described to have from 2 to 95 wt % of the first component and from 98 to 5 wt % of the second copolymer component. In the examples, the polypropylene used is Escorene® 4292, an isotactic polypropylene homopolymer having a nominal melt flow rate (MFR) of 2.0 g/10 min, and the second copolymer is illustrated by an Mw (weight-average molecular weight) of 248,900 to 318,900 and by a Mooney viscosity (ML (1+4) at 125° C. according to ASTM D1646) of from 12.1 to 38.4. The blends are directed to improved mechanical properties of processing, increased tensile strength, elongation, and overall toughness.
Despite the improvements offered above, there is a continuing need for soft elastic fabric that is capable of being stretched while retaining overall integrity, one that is useful for various components in, for example, baby diapers, pull-up pants, adult incontinence products, disposable garments, and other coverings.